1. Field of the Invention
The present invention relates to a catalyst for reducing mercury, a mercury conversion unit and an apparatus for measuring total mercury in exhaust gas by using the same, and relates in particular to a catalyst for reducing mercury, a mercury conversion unit and an apparatus for measuring total mercury in exhaust gas by using the same, wherein total mercury in coal combustion exhaust gas for example is measured.
2. Description of the Related Art
As an apparatus for measuring total metal mercury in combustion exhaust gas, there has been conventionally used an apparatus for measuring total mercury for a fixed source by using a continuous measurement method or a dilution measurement method of using a gold amalgam catching/concentrating operation, stipulated under JIS K 0222. The dilution measurement method of using gold amalgam is a method of measuring metal mercury, which comprises heating a sample gas at high temperatures to reduce a mercury compound into metal mercury, then diluting it to catch mercury as gold amalgam, and after a predetermined time, re-gasifying amalgam mercury at high temperatures, and measuring metal mercury by a ultraviolet absorption method (see for example JIS K 0222-1997).
As applications are expanded in recent years, however, conventional methods of measuring mercury in, for example, exhaust gas from combustors are influenced by the presence of nitrogen oxides (NOx), sulfur dioxide (SO2) or hydrogen chloride (HCl) in the exhaust gas, and thus it is difficult to obtain sufficiently accurate measurement values. At the request of improvement of measurement methods or of new measurement methods, the following various proposals are made at present.
Specifically, as shown in FIG. 7, there is proposed a method of continuously analyzing gaseous total mercury contained in exhaust gas upon treatment of sludge and wastes, wherein a mercury-containing gas is heated (about 230° C.) if necessary and then the mercury-containing gas is treated in a gaseous form with a heated (about 200° C.) solid reduction catalyst 21 consisting of a metal (metal tin, metal zinc etc.) thereby reducing a mercury compound (mercury chloride, mercury oxide etc.) in the mercury-containing gas into metal mercury which is then measured with a flameless atomic absorption spectrometer 22 (see, for example, JP-B 1-54655).
In an apparatus 31 for analyzing mercury in a mercuric chloride-containing gas, as shown in FIGS. 8(A) and (B), a reducing agent 34 comprising a stannous chloride coating 33 formed on the surface of tin particle 32 is charged into a reduction reactor 35, and by a reduction apparatus 36, the gas is passed through the reduction reactor 35, whereby Hg2+ in mercuric chloride is reduced to Hg0 by the reducing agent 34, and the reduced Hg0 is analyzed by an analyzer (flameless atomic absorption spectrometer) 37. By doing so, mercury analysis can be properly carried out even if the concentration of mercuric chloride in the gas is low (see, for example, JP-A 2001-33434).
However, when the measurement methods or measuring apparatuses described above are used to measure total mercury in coal combustion exhaust gas, accurate measurement is difficult because of poisoning of the catalyst by metal oxides such as selenium oxide and arsenic oxide (both of which are gases) coexisting in exhaust gas and the influence of coexisting gas components SO2, NO2 and water on the catalytic activity.
That is, it was found through the inventors' verification that in a process of reduction treatment of mercury compounds (divalent mercury), metal oxides undergo reduction reaction simultaneously with the reaction of the mercury compounds, to easily form amalgam with mercury and catch mercury, thus gradually lowering measurement values and making mercury measurement infeasible in some cases. Particularly, coal combustion exhaust gas contains a relatively large amount of metal oxides such as those of lead (Pb) and selenium (Se) which easily form amalgam with mercury, and their influence cannot be negligible and their avoidance is difficult by the conventional methods.
The dilution measurement method of using a gold amalgam catching/concentrating operation prescribed in JIS K 0222 supra has problems such as significant errors in dilution, limitation to batch measurement, and deterioration in performance of high-temperature reduction catalyst. This conventional method makes use of a high-temperature catalyst, but there is also a problem of necessity for arrangement of an acid scrubber because SO2 is oxidized at high temperatures to form SO3 mist. Further, element mercury is easily oxidized again with gas-contacting materials (for example, stainless steel (SUS)) used for the high-temperature catalyst, so the selection of a material constituting the catalyst unit is necessary.
As described above, there are some demands for the apparatus for measuring total mercury, which is directed to coal combustion exhaust gas, but the apparatus for continuously measuring mercury by an extraction sampling system other than the dilution method of using a gold amalgam/concentrating operation is substantially not developed under the present circumstances.
In the atomic absorption spectrometry, photoabsorption in the ultraviolet range is utilized, and thus the interference influence of SO2 and NO2 coexistent at a high concentration of several thousand ppm in coal combustion exhaust gas cannot be negligible.
To cope with such demand, the object of the invention is to provide a catalyst for reducing mercury and a mercury conversion unit, which can maintain high reducing function even if various metal oxides and strongly acidic corrosive gases are coexistent such as in coal combustion exhaust gas. The present invention also provides an apparatus for measuring total mercury comprising the catalyst for reducing mercury and the mercury conversion unit, which is capable of continuous measurement with high accuracy and stability for a long time without undergoing the influence of coexisting components.